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Full text of "Handbook Of Chemical Engineering - I"

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Size of Plant.—In determining size of plant, one of the first considerations is the ratio of load fluctuations (or some load function; kilowatts, pounds of steam, pounds of coal, outdoor temperature and so on) to average load during short periods of time. In Fig. 2 the graph is such as might be obtained from a recording (indicating) instrument like an ammeter or water meter or steam-flow meter. The horizontal "average" load would be simultaneously obtained by dividing the work done (or its function) by time. Assume 50,000 kw.-hr. to be the work done in 5 hr. Theri the dotted line represents the average load of 10,000 kw. The variations from this, in Fig. 2, show whether the plant capacity may safely be adjusted to suit short-period average loads (as given in Fig. 1) or whether the fluctuations must be considered. In general, overload capacities will suffice to care for reasonable fluctuations. Such " swings" as are shown in Fig. 2 are rare, when the entire plant is considered, although they may occur on individual machines. Gas and oil engines have very slight overload capacities, and if used on a load like that shown, the plant should have a capacity equal to the maximum rather than the average load.
Size of units is the next point to be decided. If a unit is to be held in reserve at all times, then the smaller the size of unit the smaller is the total investment. The size should be governed by the load curve (Fig. 1). More than one size may be used. Thus if Fig. 1 represents an average day, 700 kw. would be sufficient capacity for the period from 11 p.m. to about 6 a.m. During the time from 9 a.m. to 5 p.m., about 1,800 kw. would be needed. Units of 700 kw. and 1,100 kw. might be selected, with another 700 kw. unit in reserve, but it would be safer if the reserve unit were 1,100 kw. Some attention must be given the question of capacities commercially obtainable. The load curve should be observed for many days. Exceptional conditions like Sunday service should be considered. Provision of reserve equipment is far less the general practice than was formerly the case. In order of reliability, steam, producer gas and oil follow one another about in the order named. Waterwheels are very reliable of themselves, but the dependability of their service hangs on other factors. Purchased power from large central or transmission plants is usually highly reliable, but there may be local considerations. Reserve boilers or producers should always be provided to give opportunity for cleaning, etc.
Constant and Variable Costs.—In the illustration discussed, the total capacity of plant selected will be 2,500 kw. or 2,900 kw. as the case may be, although the average load is only 1,219 kw. Hence, the capital charges will be relatively high. The higher the load factor, the lower the rate of capital charge. For a given plant, the larger the output, the less is the "overhead" expense (interest, depreciation, taxes, insurance) per unit of output. Figure 3 shows the general law for all types of plant: the total cost is made up of a constant part plus a variable part which has a linear relation with the load. Figure 4 shows corresponding variations of cost per kilowatt-hour with load in kilowatts. Curves like those of Figs. 3.and 4 will also represent steam and coal consumption, labor cost, etc. The constant cost of these two diagrams includes not only overhead charges but also the stand-by fuel consumption when the plant is idle, non-liquid labor and maintenance charges, etc.
If the hourly constant costs are once known, and the total cost at any particular load is also known, the Klingenberg chart (Fig. 5) is useful. In the quadrant (7) the ordinate to any point of the curve represents the minimum load carried through-